1. Field of the Invention
The present invention relates to an optical disc (for example, compact disc (CD), DVD, etc.) drive. More specifically, the present invention discloses an optical disc drive with a high frequency compensator for preventing data access failure caused by wobbles of the disc.
2. Description of the Prior Art
With current technology, CDs have the characteristics of being light weight, having a small physical volume, and having a low cost. In addition, CDs have a high capacity for information storage, making CDs an indispensable information-storing medium.
Please refer to FIG. 1. FIG. 1 is a perspective view of a prior optical disc drive 10. The optical disc drive 10 comprises a housing 12 and a rotatable platform 16 installed on the housing 12. The platform 16 is designed for a CD 14 to be placed thereon. Additionally, the housing 12 has a hole 17 inside the housing 12. A sled 18 slides along a direction of the hole 17 for accessing the data stored on the CD 14.
Please refer to FIG. 2. FIG. 2 is a diagram of the internal mechanisms of the prior optical disc drive 10. For accessing data stored on the CD 14 in high-density, the optical disc drive 10 has control circuitry 38 for controlling the optical disc drive 10. The housing 12 of the optical disc drive 10 is omitted in FIG. 2 to show internal structures clearly, and only a portion of the CD 14 is shown in FIG. 2. The optical disc drive 10 has a motor 15 disposed inside the housing 12 for driving the rotation of the platform 16 and further driving the rotation of the CD 14 on the platform 16. The sled 18 slides along a track 30 inside the housing 12 (slides along directions 34) for accessing the data stored on the CD 14. The sled 18 has an actuator 22 sliding along directions 35, and an optical component 32 (such as a lens) fixed on the actuator 22. Additionally, a light source 26 disposed on the sled 18 is coupled to the actuator 22 via an optical path.
The data access operations can be described as follows. Light emitted from the light source 26 of the sled 18 is guided onto the actuator 22 through the optical path (commonly, the optical path comprises a reflection or a refraction process of a reflector and a prism), and focused onto a bottom surface of the CD 14 by the optical component 32. The light signal, which is modulated and reflected by the CD 14, is reflected back to the optical component 32 of the actuator 22. The optical disc drive 10 accesses the data stored on the CD 14 by analyzing the reflection light.
The light generated from the light source 26 must be accurately focused onto the CD 14 by the optical component 32 of the actuator 22 for assuring accuracy of data access. Therefore, the optical disc drive 10 can exactly analyze the light reflected from the CD 14 and further access data stored on the CD 14. The optical component 32 has a certain focus range, so the actuator 22 must keep within a certain vertical distance from the CD 14 so as to enable light to be accurately focused onto the CD 14. In order to control the actuator 22 to ensure the accuracy of focus, the control circuitry 38 of the optical disc drive 10 comprises a focus compensation device 48, and the sled 18 also comprises a sensor 28 optically coupled to the actuator 22.
The light generated from the light source 26 and reflected from the CD 14 to the optical component 32 is analyzed by the optical disc drive 10 to access data stored on the CD 14. Furthermore, a portion of the reflected light is incident to the sensor 28 through the optical path. The sensor 28 analyzes the reflected light and generates an error signal, which indicates a focus error of the light from the optical component 32 onto the CD 14. The error signal generated by the sensor 28 is transmitted to the control circuitry 38. When magnitude of the error signal is greater, the focus error is greater. In addition, a positive or a negative symbol of the error signal indicates that the distance between the actuator 22 and the CD 14 is either too near or too far. The focus compensation device 48 inside the control circuitry 38 controls the actuator 22 to wobble upward and downward. The focus compensation device 48 changes the vertical distance between the actuator 22 and the CD 14 according to the error signal, and further reduces the focus error.
The optical disc drive 10 uses the focus compensation device 48 to control the vertical location of the actuator 22 and reduce the focus error. But, the prior focus compensation device 48 is used to compensate for the focus error due to disturbance at low frequencies. Besides, the focus error results from that the CD 14 cannot rotate on a perfect plane without vertical disturbance due to the unevenness of its structure. An ideal optical disc has a perfect circular disc surface. When the motor 15 rotates the ideal optical disc, the actuator 22 can keep a certain distance with the surface of the optical disc without wobble upward and/or downward. However, in reality, the optical disc has a curved or scraggy surface caused by the manufacturing process. When the motor 15 rotates such a curved or scraggy optical disc, the vertical distance between the scraggy surfaces of the optical disc and the sled 18 inconstantly changed as the optical disc rotates. As the disc is read, the focus compensation device 48 controls the actuator 22 to wobble in synchronization with the wobble of the optical disc so that the light is accurately focused onto the optical disc by the optical component 32 of the actuator 22.
As the optical disc is rotated, the uneven portion of the optical disc causes a periodic variation of the vertical distance between the optical disc and the sled 18. To account for this variation, the focus compensation device 48 controls the actuator 22 to wobble upward and downward so as to keep the vertical distance between the optical disc and the sled 18 constant according to the periodic variation. As known, the higher the data access rate is, the higher the rotation speed of the motor 15 is. Thus, the frequency change of the vertical distance is very high. The control response of the focus compensation device 48 cannot solve problems resulted from high frequency variation. Instead, the prior optical disc drive 10 reduces the rotation speed of the motor 15 to control the focus error better. Obviously, such prior method cannot meet requirements for a high data access rate.